This invention relates to a fuel cell and more particularly, to a cell structure of a fuel cell stack.
FIG. 1 is a partial cross-sectional view illustrating a typical cell structure of a fuel cell which is disclosed, for example, in Japanese Patent Publication No. 58-152 and others. In this figure, electrodes 2 and 3 are disposed on both faces of an electrolyte matrix 1, and are comprised of electrode substrates 4 and 5 and electrode catalyst layers 6 and 7, respectively. These catalyst layers 6 and 7 are formed by coating a catalyst in a paste or liquid state on the substrates 4 and 5 and are thus supported thereon. The electrode substrates 4 and 5 are made of a porous material such as carbon fiber. One example of a method of producing the electrodes is disclosed in Japanese Patent Laid-Open No. 57-168473.
At both faces of both ends of the matrix 1 (only one end being shown) wet gas seals 8 and 9 are disposed to be adjacent to the end surfaces of the substrates 4 and 5, respectively. These wet gas seals 8 and 9 prevent the fuel and oxidant gases in the gas channels 11 and 12 from leaking through the porous substrates 4 and 5 to the outside of the fuel cell. Gas separators 10 (also called interconnectors) made of materials such as impermeable dense carbon plate are disposed at the back faces of the substrates 4 and 5 and wet gas seals 8 and 9. On the portions of the gas separators 10 adjacent to the substrates 4 and 5, gas channels 11 and 12 are provided for the fuel and oxidant gases, these channels crossing each other orthogonally.
In operating the fuel cell, fuel and oxidant gases are supplied to the gas channels 11 and 12 and then reach the whole area of the substrates 4 and 5, where the gases are diffused to reach the catalyst layers 6 and 7. Then, the fuel and oxidant gases at the catalyst layers 6 and 7 react with each other and generate power through the electrolyte matrix 1. At this time, non-reacted excess gases and water vapor, which is a reaction product, are exhausted to the exterior of the fuel cell through the gas channels 11 and 12. Moreover, the exhausted gas will contain an electrolyte vaporized from the matrix 1 and electrodes 6 and 7 which is determined by the operating conditions of the fuel cell.
However, in a conventional fuel cell as described-above, when the catalyst is coated on the substrates 4 and 5, the catalyst permeates into the substrates, because the catalyst is coated in a paste or liquid state and the substrates are porous. Since the permeated volume within each substrate 4 and 5 or between the substrates 4 and 5 are different, the coated catalyst volume or the thickness of the electrodes will not be uniform, making it difficult to obtain stable characteristics for each cell. The amount of the catalyst coated on the substrates 4 and 5 must be more than that actually consumed because the permeation of the catalyst requires excess catalyst for coating. Moreover, due to catalyst permeation, when an internal reservoir is provided on ribbed electrodes, it is difficult to estimate the volume of the internal reservoir and still obtain the expected cell performance. Further, from a productivity standpoint, a plurality of processes, such as providing the catalyst layers 6 and 7 on the substrates 4 and 5 and laminating the substrates 4 and 5 to the electrolyte matrix 1, raises the proportion of defective fuel cells and adversely affects productivity.